The invention relates to a method of controlling casting parameters in a die-casting machine. The casting parameters are controllable as a function of travel of a casting piston longitudinally displaceable in a shot sleeve.
1. State of the Art
Automatic control of die-casting machines is known from the literature reference Ernst Brunhuber: "Praxis der Druckgu.beta.fertigung [Practice of die-casting production]", 3rd edition, 1980, pages 82 et seq. According to pages 84 and 85 of this literature reference, a distinction is made between feeding phase, mold-filling phase and squeezing phase, the piston stroke of the casting piston and hence the molten metal in the shot sleeve being controlled in each phase. The quantity filled into the shot sleeve here plays a decisive role, since, for example, the start of the mold-filling phase is dependent on the quantity filled in and on the position of the casting piston.
2. The Metering of the Molten Metal is Carried Out as Follows:
In cold-chamber die-casting machines, the casting metal is taken from a holding furnace and filled into the shot sleeve of the casting unit. In manual operation, this is done by ladling the molten metal with a ladle from the holding furnace and emptying it into the filling port of the shot sleeve. Metering and charging devices have also been disclosed, by means of which the step of filling the shot sleeve with molten metal can be automated. In Brunhuber (loc. cit.), pages 105 to 110, a device is described which uses a meter ladle which dips into the holding melt, takes up an appropriate quantity of measured metal and empties the latter into the filling port, after transporting to the shot sleeve. The ladle is adjustable in such a way that it allows precise metering of the desired quantity of metal. Excess metal then flows back into the holding furnace when the ladle is moved up. The metering accuracy is indicated for a particular unit at +/-0.8% in the metering range from 0.1 to 15 kg of aluminum alloy.
In cold-chamber die-casting processes, the quality of the castings is determined by a multiplicity of process parameters. The accuracy of metering the molten metal here affects the casting quality to a high degree, and it is possible for adverse influences to affect the total casting process in the direction of an inadvertent and disadvantageous shifting of the starting points of the casting phases, of a change of the casting piston speed and of the casting pressure.
For achieving a high metering accuracy in die-casting machines, it has been disclosed by DE 84 22,336 U1 to detect the level in the die-casting mold by means of a contact pin, wherein the molten metal rising in the casting mold short-circuits two contacts and this then makes a signal. This measured signal can then be fed to a regulating or control unit and utilized for automatic control.
Also known is the use of sensors which are designed as a thermocouple and which record the temperature of the melt at a defined level.
The fitting of such sensors in the casting chamber itself frequently leads to unreliable results, since these are considerably stressed due to the high temperatures and the rough handling in the casting chamber, so that it is hardly possible to set accurate and reproducible values.
Furthermore, a wave-like or sloshing motion of the melt frequently occurs in the shot sleeve and, due to different melt levels, this can indicate false filling values. The signal emitted accordingly corresponds not to the filling level actually present. The consequence can therefore be a corresponding maloperation of the control.
Moreover, DE 3,344,537 C1 has disclosed a method of timed metering of a quantity of molten metal in die-casting. In this method, a defined measured quantity of metal is fed to the shot sleeve via a riser from a furnace by a defined compressed-air surge from a constant pressure wave. This method requires a large expense on apparatus and continual compensation of the metering time as a function of the furnace filling and of the length of the slug on the work piece. Control of the motion of the casting piston is not provided thereby.
Furthermore, DE 2,307,846 A1 has disclosed a method for automatic take-up of molten metal, wherein the metering of the quantity of melt is effected via a weight measurement. The precise level is detected via a combined pressurization of the holding furnace and a measurement of the weight of both the casting quantity released from the holding furnace and taken up by the casting mold. Additional scanning of the casting riser or directly of the casting mold during the casting step via a photoelectric cell directed upon the casting mold is here provided, and this is connected to a control system. The casting step is then monitored via the photoelectric cell (infrared eye). After a set value has been reached, for example in the casting mold, a valve is to be opened, for example in the pressure line. This causes a sharp fall in pressure, so that sloshing-over of the molten metal is avoided.
This arrangement also relates to automatic metering of molten metal in the mass production of castings. It does not relate to any control of the casting parameters as a function of the molten metal in a shot sleeve of a cold-chamber die-casting machine.